1. Field of the Invention
This invention relates to a control drive circuit, in particular to a technique that produces a drive output of a voltage that is higher than a power source voltage for the drive.
2. Description of a Prior Art
There are known control drive circuits for use in the servo-control of motors, such as one disclosed in a Japanese patent KOKOKU publication No. Hei-8-23786. FIG. 22 shows the constitution of such a control drive circuit.
The control drive circuit shown in FIG. 22 comprises; a differential amplifier 4, an output circuit 6, and an output synthesizing circuit 8. Signals from a Hall element 2 for detecting motor rotation, etc. are inputted to the differential amplifier 4. Input signals (at the point (a) in FIG. 22) to the differential amplifier 4 are shown in FIG. 23A.
Operation current that affects the amplification gain of the differential amplifier 4 is determined with a current I0. Therefore, the differential output from the differential amplifier 4 consists of the alternate current component from the Hall element 2 superimposed with the direct current component corresponding to I0/2.
The output circuit 6 is constituted to output a current that is equal to the differential output of the differential amplifier 4 minus the direct current component I0/2. The output signals (at points (b) and (c) in FIG. 22) of the output circuit 6 are shown in FIGS. 23B and 23C.
The output synthesizing circuit 8 synthesizes output signals of the output circuit 6, and outputs resultant signals in an absolute value waveform. The output waveform (at the point (d) in FIG. 22) of the output synthesizing circuit 8 is shown in FIG. 23D.
As described above, the use of the control drive circuit shown in FIG. 22 makes it possible to eliminate the influence of the current that affects the amplification gain of the differential amplifier 4 and to obtain the absolute value waveform by extracting only the alternate current component coming from the Hall element 2. Therefore, it is possible to easily obtain the absolute value waveform also with integrated circuits that are required to control the amplification gain with only direct current circuits. Another advantage is that, since no capacitors are used for removing direct current components, there is no restriction on the frequency of processing signals.
However, the conventional control drive circuit technique as described above has the following problem: Although the conventional technique as described above can provide a control drive circuit in a compact size for producing a stabilized voltage for driving a motor or the like to produce a stabilized torque, the circuit is incapable of producing a voltage that is higher than the power source voltage for driving the motor or the like.